I know there are similar questions out there, but please consider mine before writing it off as a duplicate.
I have a true/false that the View Controller receives from user input and uses to adjust the selected state of a button, then passes the true/false data to a UIView where a method is run. Once the method is done I would like to return the selected state of the button back to NO, however when I try to tell the UIView to access the ViewControllers button I run into some problems.
Could someone please tell me how I might do this and, if it's not possible, could you outline where I have strayed from the MVC pattern and how I might get back on track?
I would use an NSNotification. Easier to get your head around than delegation if this is all new to you.
register something like this in your parent view controller:
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(methodToChangeButtonState) name:#"changeButtonState" object:nil];
and call it at the appropriate time from your child viewcontroller:
[[NSNotificationCenter defaultCenter] postNotificationName:#"changeButtonState" object:nil];
Your methodToChangeButtonState will fire when the notification is posted updating your button state.
Related
I used to addObserver in viewDidLoad: and removeObserver in dealloc:. Code:
- (void)viewDidLoad
{
[super viewDidLoad];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(refreshData)
name:AnyNotification
object:nil];
}
- (void)dealloc
{
[[NSNotificationCenter defaultCenter] removeObserver:self
name:AnyNotification
object:nil];
}
But according to some articles said, it's better to addObserver in viewDidAppear: and removeObserver in viewDidDisappear:. Code:
- (void)viewDidAppear:(BOOL)animated
{
[super viewDidAppear:animated];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(refreshData)
name:AnyNotification
object:nil];
}
- (void)viewDidDisappear:(BOOL)animated
{
[super viewDidDisappear:animated];
[[NSNotificationCenter defaultCenter] removeObserver:self
name:AnyNotification
object:nil];
}
So, what's the better way to addObserver/removeObserver?
this depends on your scenario, usually the best approach is to add in viewDidLoad and remove in dealloc and in viewDidUnload (deprecated in iOS 9.0, use dealloc only), but there are some cases when you have same method in different classes like UI effects and want to call only current screen's method using notification, then you will have to add the observer in viewWillAppear and remove it in viewWillDisappear or viewDidAppear/viewDidDisappear
Edit:
A note from comments, thanks #honey.
Though now since iOS 9, you no longer need to care about removing the observer. See Apple release notes: "In OS X 10.11 and iOS 9.0 NSNotificationCenter and NSDistributedNotificationCenter will no longer send notifications to registered observers that may be deallocated..
I would normally put it in -viewDidAppear: and -viewDidDisapear: (or -viewWillAppear: and -viewWillDisappear:) simply because in every case I came across I'm only interested in the notification if the view is actually displayed.
It's probably a premature optimisation (your code for handling the notification could take some time, but might be useless if the view is not displayed), but then it's also no more code - it's the same code just in a different method...
Don't forget NSKeyValueObservingOptionInitial. I use it with viewWillAppear/viewWillDisappear so my UI is always up-do-date, even if I hide that view controller, saving resources because I will not update it until is shown again.
The best approach using NSNotifications is adding the observer when you need to observe for notifications and remove them when you don't need them anymore.
This could be on viewDidLoad:, viewWillAppear:, or when the user taps some button etc.
I will give you a little example:
My app has a tabbar, and in some of the view controllers, I'm displaying some info downloaded from internet (a tweet for example). I also have a class pooling for new data from server each 2 minutes, and as the server had new data, I updated the info on database. I will not use a delegate pattern to listen to DB changes, because I have so many view controllers displaying data, and it will be a very bad design making the delegate an array and looping to pass the data for every view controller. So, in this specific scenario, the best to do is to post a notification telling every VC that new data has come.
If your VC removes the delegate when the view disappears, only the current one will receive the notification and update the displaying contents.
You obviously could update the contents of the other VCs before display, on viewWillAppear: for example, but doing this the VC contents will be updated not only when necessary, but each time you change tabs.
It was only one example, where I tried to show you that for NSNotifications, is difficult to advise you when to add or remove observers when we don't have the entire description of how you app behaviours.
-viewWillAppear: + -viewWillDisappear: is better than -viewDidAppear: + -viewDidDisapear:, because they are always called the same number of times.
I want to pause a timer on my game screen when the iPhone is locked etc. My question is what is the best method to notify the current UIView, which the AppDelegate has no direct access to?
1) Your timer should probably not be managed by the view but by the view's controller. The timer itself is not an inherent part of your UI, only the timer's display is. (What happens if you want to have the timer continue after a view is removed, for example?)
2) Any object (view or controller included) can independently listen for the appropriate notification. For example, in your view controller (or view code, if you choose to go that route):
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(pauseTheTimer:)
name:UIApplicationWillResignActiveNotification
object:nil];
Then implement a pauseTheTimer: method that will handle the notification. (Since there is only one UIApplication object, you can use nil for the object, as shown.)
This approach nicely decouples your app delegate from the views and view controllers.
(Oh, don't forget to stop observing when your view is unloaded or deallocated. Failure to do so can and will lead to crashes.)
here i am again:
what i want to do is:
if i press a button, then post a notification. This notification should be cached by 2 instances of the same class.
the problem:
the notification is posted, but it is cached just by one instance.
some code and explanation
i have 1 tab bar controller
i have 3 tabs ( 3 different views -xib files-)
2 views references the same (view controller) class (so, there are 2 instances of the same class, let's say class A)
the other tab/view references another class (class B)
if i press a button of one view, a method of class B is fired and, at some point it does this:
[[NSNotificationCenter defaultCenter] postNotificationName:#"update" object:nil ];
in the viewDidLoad method of class A I have this:
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(updateAll:) name:#"update" object:nil];
also, i have defined the updateAll function as:
- (void) updateAll: (NSNotification *) notification {
NSLog(#"called");
}
As i said before, just one time the updateAll method is fired.
questions
why?
how to fix it?
thanks for reading!
It is possible that your view is not loaded yet, because you are using tab bar controller. The view that is not yet visible is not loaded, so it is likely that your viewDidLoad will get called only for one instance. I recommend you debug it and make sure your addObserver call is really get executed twice, not once.
This won't work at all. You're posting a notification with a name #"updated" but you've attached observers for name #"update". You should be getting no notifications at all.
The way of posting notification is synchronous. I think another object doesn't register as an observer yet, so it cannot receive the notification posted.
And, if the notification is posted on another thread, it will be obtained by the observer on the same thread.
What are the possible ways to send data to previous view in iphone. Without using Appdelegate. Because there are chances for my view class to be instantiated again.
I believe the best approach is using the NSNotificationCenter class.
Basically what you do is register an object (as an observer) with a notification center.
So for example if you have objects A and B. A registers as an observer. Now lets say A is the "previous" object you are talking about, you can have B send a notification (data or message) to the notification center which then notifies object A (and any other registered observers).
Example:
In file ClassA.m register as shown below:
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(didSomething:) name:#"SomethingHappened" object:nil];
didSomething is the method which receives the notification sent by object B. This will look something like
- (void) didSomething: (NSNotification *) notify {
...
}
Finally you send the message below from whatever method in ClassB.m to notify/send data to object A
[[NSNotificationCenter defaultCenter] postNotificationName:#"SomethingHappened" object:self userInfo:your_data];
Seems convoluted but it's the best approach in my opinion (and quite simple once you understand it :)).
There are several ways to achieve data sharing, with Singleton Objetcs being one of the most popular:
Objective C Singleton
If the view you want to communicate with is a parent view (e.g. the previous view's view controller is where you created this view) then you probably want to handle dismissing the view in the previous view controller. When you do that, you can read the data that has changed and update the previous view controller with the new data.
Then in the viewWillAppear: method of the previous view controller, update the actual views to reflect the current state of the view controller.
Edit: I've just noticed that your newView is transparent. If this is the case, then you certainly want to route all logic through your view controller. You should only have one view controller with visible views at a time.
I am using a navigation based application for iPhone. I want to reload a view when i come back to it after pressing the back button. ANy solution?
Regards
Add a method viewWillAppear: to your controller class. In that method you can then update the view with current data.
The viewWillAppear: method will execute whenever the view is about to be displayed (after navigating to a different view using UINavigationController)
There is more than one, but I usually use NSNotificationCenters. You attach "listeners" for some kind of event, like this:
[[NSNotificationCenter defaultCenter]
addObserver:self
selector:#selector(onSomethingChanged:)
name: #"somethingChangedEvent"
object: nil];
So, if some other view changes a setting, it notifies all the listeners like this:
[[NSNotificationCenter defaultCenter] postNotificationName: #"somethingChangedEvent" object: Nil];
Pretty simple and intuitive.